Molten metal furnace have tap holes therein to empty the furnaces at the conclusion of the metal heat. These tap holes are plugged with anhydrous tap hole clay which becomes very hard. In order to empty the furnaces, the plugged tap holes must be bored through by a drill to allow the moltern metal to pass therethrough.
Rotary percussion drills are used to bore through the anhydrous tap hole clay. The rotary percussion drills have a long slide mechanism associated therewith mounting a drill rod and drill bit for both rotary movement and hammering or percussion movement. The drill bit is made from carbide or other hardened metal and the drill rod is made from drill steel. When the drill bit penetrates the clay plug, the molten metal passes over and around the drill bit and drill rod to destroy or severly damage the same. This destruction or damage results in the relatively expensive drill bit and drill rod having to be replaced for substantially every heat of the furnace.